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Hurricane Oscar (2018)
Hurricane Oscar was the first Atlantic major hurricane ever recorded in the month of December. The fifteenth and final named storm of the 2018 Atlantic hurricane season, Oscar formed along the southern end of a very deep cold front in the southwestern Caribbean Sea. Amidst a small pocket of low shear, Oscar rapidly intensified and became a major hurricane. After skirting the coast of Nicaragua where flooding rains from Oscar caused moderate damage. Due to increasing shear from another cold front which wrapped into Oscar's circulation, the system transitioned into a deep extratropical cyclone and underwent bombogenesis over the eastern Gulf of Mexico. As the eastern United States was embedded in a very cold, dry air mass, Oscar dumped heavy, wet snow over much of the eastern United States from central North Carolina to Maine. After transitioning into an extratropical cyclone, Oscar was referred to by many in the media as Superstorm Oscar or Nor'Oscar. Meteorological History The development of Oscar was complex. On November 29, a strong cold front passed through the eastern United States and moved offshore. A weak area of low pressure developed along the southern end of the front over the southwestern Caribbean late that day. A weak late-season tropical wave, which had exited the coast of western Africa on November 23, interacted with the low pressure system, and deep convection began to organize. A strong subtropical ridge built in over the eastern United States, which brought unseasonably cold and dry weather to the region. Although wind shear over the Caribbean had been persistently high since mid-October, a small favorable area opened up near the disturbance due to an upper-level anticyclone forming aloft. This allowed the low pressure system to organize and become more concentrated, and NHC began monitoring it for potential development early on November 30. It was initially anticipated that the strong subtropical ridge would steer the disturbance west-northwestward into central America, which would have limited development. Due to a weakness in the ridge over the central United States, the disturbance began moving very slowly northward late that day in an area of weak steering currents. After Dvorak T-numbers from the Satellite Analysis Branch (SAB) and Tropical Analysis and Forecast Branch (TAFB) increased to T2.0 on the standard Dvorak Scale, as well as scatterometer data showing a well-defined circulation, NHC initiated advisories on the sixteenth tropical depression of the season at 09:00 UTC December 1. Operationally, it was estimated that the depression formed three hours earlier. This made Sixteen the first Atlantic December tropical cyclone since Olga in 2007. At 12:00 UTC on December 1, the depression strengthened into Tropical Storm Oscar, based on improved convective banding. Oscar quickly developed a very small inner core over warm waters of 28°C (82.4°F), and an Air Force Hurricane Hunter aircraft that afternoon confirmed the development of a primitive eyewall. Due to favorable conditions, Oscar began to rapidly intensify. After a reconnaissance aircraft measured hurricane-force winds from the SFMR instrument, Oscar strengthened into a hurricane at 12:00 UTC on December 2 as it began to move faster northward. This made Oscar the Atlantic's first December hurricane since Epsilon 13 years prior. Oscar's eye continued to warm while cloud tops surrounding the eye cooled, and Oscar strengthened into a Category 2 hurricane just six hours after first becoming a hurricane. This made Oscar the first Category 2 hurricane ever recorded in the Atlantic basin in December. Further intensification ensued, and at 06:00 UTC on December 3, Oscar briefly intensified into a Category 3 hurricane with maximum sustained winds of 120 miles per hour. This made Oscar the first Atlantic major hurricane ever recorded outside of the official bounds to the season. NHC forecaster Lixion Avila described Oscar's rapid intensification as "an unprecedented situation I never expected to happen." After peaking intensity, increasing southwesterly wind shear caused Oscar's eye to quickly disappear in visible satellite imagery, with the hurricane falling below major hurricane status just six hours after peak intensity. The shear caused the cyclone to develop a tilted structure by late December 3, with the surface circulation moving away from the mid-level circulation. By the morning of December 4, Oscar's low-level circulation had become elongated and exposed to the west of the convective mass, and Oscar fell below hurricane strength at 12:00 UTC on December 4. Due to sea surface temperatures falling to 26°C, and continued increasing shear, Oscar transitioned into a post-tropical cyclone at 00:00 UTC on December 5. Another cold front, which caused Oscar to accelerate to the north over the southeastern Gulf of Mexico, wrapped into Oscar's circulation, completing its transition to an extratropical cyclone. The low-latitude extratropical remnants of Oscar underwent a brief period of bombogenesis over fairly warm waters in the eastern Gulf of Mexico, with ex-Oscar strengthening into a hurricane-force extratropical low by 00:00 UTC on December 6. Even though Oscar had lost tropical characteristics, the Air Force Hurricane Hurricane Hunters continued to monitor the system, due to its potential to bring hurricane-force winds and heavy rainfall to the northeastern Gulf Coast. The National Hurricane Center continued to issue advisories on the system, issuing Hurricane Watches for the Northern Gulf Coast, noting that ex-Oscar had potential to bring "hurricane-like conditions" to Northwestern Florida, an area that was hit hard by Hurricane Michael just two months prior. At this time, NHC also noted the possibility ex-Oscar could bring the first significant snowfall of the season to the Appalachian Mountains, Mid-Atlantic, and New England. As a result, Winter Weather Advisories were issued for many counties. Just before 12:00 UTC on December 6, ex-Oscar's circulation moved inland near Tampa Bay, Florida. Maximum sustained winds of 71 mph were reported near St. Petersburg, and it was estimated that ex-Oscar was a 75 mph extratropical cyclone at landfall. After landfall, the Weather Prediction Center (WPC) took over advisories on ex-Oscar. As cold air continued to wrap into ex-Oscar's circulation, precipitation in the western semicircle changed from rain to snow. Category:Unusual tropical cyclones Category:Off-season storms Category:2018 Atlantic hurricane season Category:Unrealistic storms